jresponse: make to_string accept non-string arguments

This lets ruby callers pass e.g. FixNums without worrying about it.

Without it, the following exception is thrown:

     Failure/Error: output = Killbill::Plugin::JConverter.to_string(input)
     NameError:
       no constructorfor arguments (org.jruby.RubyFixnum) on Java::JavaLang::String
         available overloads:
           (java.lang.String)
           (char[])
           (java.lang.StringBuffer)
           (java.lang.StringBuilder)
           (byte[])
     # ./lib/killbill/jresponse/jconverter.rb:25:in `to_string'
     # ./spec/killbill/jresponse/jconverter_spec.rb:43:in `(root)'

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>